Ran Hu

Estimating hydraulic conductivity of fractured rocks from high‐pressure packer tests with an Izbash's law‐based empirical model

High-pressure packer test (HPPT) is an enhanced constant head packer test for characterizing the permeability of fractured rocks under high-pressure groundwater flow conditions. The interpretation of the HPPT data, however, remains difficult due to the transition of flow conditions in the conducting structures and the hydraulic fracturing-induced permeability enhancement in the tested rocks. In this study, a number of HPPTs were performed in the sedimentary and intrusive rocks located at 450 m depth in central Hainan Island. The obtained Q-P curves were divided into a laminar flow phase (I), a non-Darcy flow phase (II), and a hydraulic fracturing phase (III). The critical Reynolds number for the deviation of flow from linearity into phase II was 25−66. The flow of phase III occurred in sparsely to moderately fractured rocks, and was absent at the test intervals of perfect or poor intactness. The threshold fluid pressure between phases II and III was correlated with RQD and the confining stress. An Izbashs law-based analytical model was employed to calculate the hydraulic conductivity of the tested rocks in different flow conditions. It was demonstrated that the estimated hydraulic conductivity values in phases I and II are basically the same, and are weakly dependent on the injection fluid pressure, but it becomes strongly pressure dependent as a result of hydraulic fracturing in phase III. The hydraulic conductivity at different test intervals of a borehole is remarkably enhanced at highly fractured zone or contact zone, but within a rock unit of weak heterogeneity, it decreases with the increase of depth.

A new classification of seepage control mechanisms in geotechnical engineering

Abstract Seepage flow through soils, rocks and geotechnical structures has a great influence on their stabilities and performances, and seepage control is a critical technological issue in engineering practices. The physical mechanisms associated with various engineering measures for seepage control are investigated from a new perspective within the framework of continuum mechanics; and an equation-based classification of seepage control mechanisms is proposed according to their roles in the mathematical models for seepage flow, including control mechanisms by coupled processes, initial states, boundary conditions and hydraulic properties. The effects of each mechanism on seepage control are illustrated with examples in hydroelectric engineering and radioactive waste disposal, and hence the reasonability of classification is demonstrated. Advice on performance assessment and optimization design of the seepage control systems in geotechnical engineering is provided, and the suggested procedure would serve as a useful guidance for cost-effective control of seepage flow in various engineering practices.

Coupled hydro-mechanical analysis of a dam foundation with thick fluvial deposits: a case study of the Danba Hydropower Project, Southwestern China

The Danba Hydropower Project, located in Danba County, Sichuan Province, China, was planned to be constructed on a thick fluvial deposit foundation, with a maximum depth of the deposits up to 133 m. The loose deposits are characteristic of complex origin, composition, distribution, and mechanical and hydraulic properties. The controls of seepage flow and settlement in the foundation become two key technological issues for construction of the dam. In this study, the coupled processes between groundwater flow and deformation were modelled with the finite element method for performance assessment of the seepage control system and the foundation treatments. The saturated flow process was formulated with an adaptive variational inequality formulation of Signorini’s condition, which eliminates the singularity of the seepage points and the resultant mesh dependency. The deformation response of the loose deposits was described using the Duncan–Chang non-linear elastic E–B model, together with the Goodman interfacial elements for simulation of the soil–structure interactions. The stress-dependent variations in the permeability of the loose deposits were considered on the basis of the Kozeny–Carman’s model. The numerical results indicate that the designed seepage-control structures are effective and the settlement is limited within 20 cm using proper foundation treatments.

Visualizing and quantifying the crossover from capillary fingering to viscous fingering in a rough fracture

Immiscible fluid-fluid displacement in permeable media is important in many subsurface processes, including enhanced oil recovery and geological CO2 sequestration. Controlled by capillary and viscous forces, displacement patterns of one fluid displacing another more viscous one exhibit capillary and viscous fingering, and crossover between the two. Although extensive studies investigated viscous and capillary fingering in porous media, a few studies focused on the crossover in rough fractures, and how viscous and capillary forces affect the crossover remains unclear. Using a transparent fracture-visualization system, we studied how the two forces impact the crossover in a horizontal rough fracture. Drainage experiments of water displacing oil were conducted at seven flow rates (capillary number log10Ca ranging from −7.07 to −3.07) and four viscosity ratios (M=1/1000,1/500,1/100 and 1/50). We consistently observed lower invading fluid saturations in the crossover zone. We also proposed a phase diagram for the displacement patterns in a rough fracture that is consistent with similar studies in porous media. Based on real-time imaging and statistical analysis of the invasion morphology, we showed that the competition between capillary and viscous forces is responsible for the saturation reduction in the crossover zone. In this zone, finger propagation toward the outlet (characteristic of viscous fingering) as well as void-filling in the transverse/backward directions (characteristic of capillary fingering), are both suppressed. Therefore, the invading fluid tends to occupy larger apertures with higher characteristic front velocity, promoting void-filling toward the outlet with thinner finger growth and resulting in a larger volume of defending fluid left behind.

Wettability and Flow Rate Impacts on Immiscible Displacement: A Theoretical Model

Author(s): Hu, R; Wan, J; Yang, Z; Chen, YF; Tokunaga, T | Abstract: ©2018. American Geophysical Union. All Rights Reserved. When a more viscous fluid displaces a less viscous one in porous media, viscous pressure drop stabilizes the displacement front against capillary pressure fluctuation. For this favorable viscous ratio conditions, previous studies focused on the front instability under slow flow conditions but did not address competing effects of wettability and flow rate. Here we study how this competition controls displacement patterns. We propose a theoretical model that describes the crossover from fingering to stable flow as a function of invading fluid contact angle θ and capillary number Ca. The phase diagram predicted by the model shows that decreasing θ stabilizes the displacement for θ≥45° and the critical contact angle θc increases with Ca. The boundary between corner flow and cooperative filling for θ l 45° is also described. This work extends the classic phase diagram and has potential applications in predicting CO2 capillary trapping and manipulating wettability to enhance gas/oil displacement efficiency.

Effect of seepage control on stability of a tailings dam during its staged construction with a stepwise-coupled hydro-mechanical model

Seepage flow renders tailings dams vulnerable to failure during their staged construction. Draining is one of the most effective measures in improving dam stability. In this study, a stepwise-coupled hydro-mechanical model is employed to examine the effect of seepage control on the stability of a tailings dam during its staged construction. The settlement and deformation of the tailings under gravity load are modelled using the Duncan–Chang non-linear elastic E–B model, and the seepage flow through the tailings with drains is characterised by a variational inequality formulation of Signorini’s condition. The Kozeny–Carman equation is calibrated to illustrate the dependence of hydraulic conductivity on the porosity and volumetric deformation of the tailings. The proposed model was applied to assess the performance of the drains designed for the Luogou tailings disposal in Luanchuan County, Henan Province, China. Numerical results show that the stress-induced variation in tailings permeability could be of 1–2 orders of magnitude, and a proper design of the drainage system is of great importance in lowering the phreatic surface and protecting the tailings from seepage erosion.

A threshold stresses-based permeability variation model for microcracked porous rocks

Abstract The dependence of permeability on stress is an important property of rocks related to their microstructural alterations. Justified by the distinct regions on the stress–strain curves under mechanical loading, this study proposed a threshold stresses-based permeability variation model for microcracked porous rocks by separating voids of rocks into a more-rounded pore system and a flat crack system. In the proposed model, the permeability reduction caused by progressive closure of pre-existing cracks is characterised by a negative exponential function of the effective mean stress, while the enhancement of permeability induced by the initiation and growth of new cracks is correlated to the cumulative acoustic emission counts described with a normal cumulative distribution function. The permeability induced by the elastic compaction of the less-compressible pores is assumed to decrease linearly with effective mean stress, typically with a rather small slope, which lower bounds the permeability of rocks when the pre-existing cracks are sufficiently closed while new cracks are not yet initiated. The proposed model has advantages of clear physical meaning, easy laboratory parameterisation and low computational cost and reproduces well the main features of permeability variation observed in the laboratory for sedimentary rocks under hydrostatic loading and granitic rocks under triaxial loading.